Conventional vehicles include an internal combustion engine that provides all or a portion of motive power for the vehicle and a battery that provide electrical power to various electric systems in the vehicle including a starter motor for the internal combustion engine. Because of the need to maintain sufficient power for key vehicle components such as the starter motor, the state-of-charge (SOC) of the battery is carefully monitored and power delivery from the battery is controlled to maintain a sufficient SOC. A generator is coupled to the internal combustion engine and, during operation of the engine, provides electrical power to vehicle electrical systems in order to reduce power demands from the battery.
The generator includes a voltage regulator. The voltage regulator can be controlled to implement a variety of different operating modes for the vehicle. In a fuel economy mode, for example, the voltage regulator reduces the voltage output of the generator to reduce the amount of fuel required by the internal combustion engine and thereby conserve fuel. Because the voltage output of the generator is reduced, additional power demands are placed on the vehicle battery. One of the conditions for entering fuel economy mode may therefore be that the battery has a sufficient SOC (e.g., more than 80%).
The battery must maintain a sufficient SOC to power the starter motor. The SOC required to power the starter motor can vary with changes in the weather. In particular, when ambient air temperatures are low, a greater SOC may be required to power the starter motor. Because operating in fuel economy mode places additional power demands on the vehicle battery, the SOC of the battery may be reduced to a level that is insufficient to power the starter motor during a subsequent restart of the vehicle.